This invention relates to polyethylene compositions that are new and useful, and films produced therefrom that are characterized by preferably having increased moisture vapor transmission rates (MVTR).
It is known that properly formulated films fabricated from polyethylene that have been filled with solid particles (e.g., a mineral such as calcium carbonate) can be stretched under appropriate conditions creating a porous structure that allows for the diffusion of water vapor while providing a liquid barrier. This is a desirable property in markets such as hygienics, industrial and medical. Indeed, a major consideration in the comfort of a garment is its ability to maintain a balance between heat production and heat loss. The loss of heat through clothing may occur through direct dry heat loss or by moisture evaporation. In respect to the latter, the moisture vapor transmission rate of the material utilized in forming the garment is generally related to the breathability of the material. Breathability is the ability to diffuse moisture/water vapor through a film or garment. In addition to this property, there are many applications requiring that the material used in preparing the garment be impermeable to a liquid. Such applications include diaper back sheets, sanitary napkins, medical protective garments, surgical incise drapes, transdermal patches, wound care bandages and dressings, intravenous site dressings, ostomy site dressings, breathable housewrap, among others.
Films which are permeable to water vapor and are porous but yet are intended to be impermeable to a liquid are described in U.S. Pat. Nos. 4,626,252 and 5,073,316. As disclosed, a porous film is obtained by mixing a polyolefin resin, an inorganic filler and a plasticizer; forming a film from the mixture; and uniaxially or biaxially stretching the film. Films of this type are also disclosed in U.S. Pat. No. 5,998,505 and PCT International Application Publication No. WO 98/05501.
It is accordingly an object of the present invention to provide new and improved polyethylene compositions or blends.
It is a further object of this invention to provide novel polyethylene compositions or blends of specific polyethylenes and fillers.
It is a further object of this invention to provide novel films produced from the novel compositions, which films are characterized by having improved moisture vapor transmission rates (MVTR).
These and other objects and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and claims.
In accordance with the present invention, it has been found that the above and still further objects are achieved by combining at least one or more specific polyethylenes and a filler to provide a new and novel composition that is suitable to provide films that are characterized by having improved moisture vapor transmission rates (MVTR). The compositions and films are useful in many applications.
More particularly, in accordance with the present invention, a polyethylene composition is provided comprising (a) an ethylene homopolymer or ethylene interpolymer having a density of from about 0.91 to about 0.93 g/cc (grams/cubic centimeter), a melt index (M.I.) of from about 1 to about 5 grams per 10 minutes (g/10 min), a weight percent high temperature fraction (% HT) as determined by TREF of about 25 to about 50 weight %, and a number average molecular weight (Mn) of the HT fraction collected during TREF procedure of from about 35,000 to about 52,000 g/mol, with the % HT preferably ranging from about 30 to about 45%; preferably the ethylene homopolymer or interpolymer component is present in the composition in an amount of from about 20 to about 80 weight percent (%); and (b) a filler present in an effective amount, such that a film formed from the novel polyethylene composition has increased MVTR; preferably the filler is present in the composition in an amount of from about 20 to about 80 weight percent.
In addition to the novel compositions, the present invention is also directed to films formed from the novel compositions that are characterized by having increased moisture vapor transmission rates.
Additionally, the present invention is directed to articles of manufacture incorporating the novel compositions and novel films, of the present invention. Such articles include garments, diapers, sanitary napkins, medical protective garments, surgical incise drapes, transdermal patches, wound care bandages and dressings, intravenous site dressings, and ostomy site dressings, and others, incorporating the novel thermoplastic compositions and films of the present invention.
The compositions of the present invention comprise (a) at least one, or more, of an ethylene homopolymer or ethylene interpolymer having a density of from about 0.91 to about 0.93 g/cc (gram/cubic centimeter), a melt index (M.I.) of from about 1 to about 5 grams per 10 minutes (g/10 min), a weight percent high temperature fraction (% HT) as determined by TREF of about 25 to about 50 weight %, and a number average molecular weight (Mn) of the HT fraction collected during TREF procedure of from about 35,000 to about 52,000 g/mol, with the % HT preferably ranging from about 30 to about 45%; preferably the ethylene homopolymer or interpolymer component is present in the composition in an amount of from about 20 to about 80 weight percent (%); and (b) a filler present in an effective amount, such that a film formed from the novel polyethylene composition has increased MVTR; preferably the filler is present in the composition in an amount of from about 20 to about 80 weight percent.
The ethylene component of the present composition is a homopolymer of ethylene or an interpolymer of ethylene and at least one or more other olefin(s). Preferably, the olefins are alpha-olefins. The olefins may contain from about 2 to about 16 carbon atoms. The interpolymer of ethylene and at least one other olefin comprise an ethylene content of at least about 50% by weight of the total monomers involved. Exemplary olefins that may be utilized herein are propylene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 4-methylpent-1-ene, 1-decene, 1-dodecene, 1-hexadecene and the like. Also utilizable herein are polyenes such as 1,3-hexadiene, 1,4-hexadiene, cyclopentadiene, dicyclopentadiene, 4-vinylcyclohex-1-ene, 1,5-cyclooctadiene, 5-vinylidene-2-norbornene and 5-vinyl-2-norbornene.
The ethylene homopolymer or interpolymer of the present composition may be prepared by any manner known to those skilled in the art. In the present instance, the ethylene homopolymers and interpolymers were prepared using the following procedure.
The polymerization process utilized herein was carried out in a fluidized-bed reactor for gas-phase polymerization, consisting of a vertical cylinder of diameter 0.74 meters and height 7 meters and surmounted by a velocity reduction chamber. The reactor is provided in its lower part with a fluidization grid and with an external line for recycling gas, which connects the top of the velocity reduction chamber to the lower part of the reactor, at a point below the fluidization grid. The recycling line is equipped with a compressor for circulating gas and a heat transfer means such as a heat exchanger. In particular the lines for supplying ethylene, an olefin such as 1-butene, 1-pentene and 1-hexene, hydrogen and nitrogen, which represent the main constituents of the gaseous reaction mixture passing through the fluidized bed, feed into the recycling line. Above the fluidization grid, the reactor contains a fluidized bed consisting of a polyethylene powder made up of particles with a weight-average diameter of about 0.5 mm to about 1.4 mm. The gaseous reaction mixture, which contains ethylene, olefin comonomer, hydrogen, nitrogen and minor amounts of other components, passes through the fluidized bed under a pressure ranging from about 280 psig to about 300 psig with an ascending fluidization speed, referred to herein as fluidization velocity, ranging from about 1.6 feet per second to about 2.0 feet per second. The reactor temperature generally ranges from about 30 to about 110xc2x0 C.
The Ziegler-Natta catalyst used was obtained from Grace Davison, Baltimore, Maryland, under the product name XPO-5021. The catalyst was a titanium-based catalyst supported on silica. The catalyst was introduced directly into the reactor without having been formed into a prepolymer. The rate of introduction of the catalyst into the reactor was adjusted in achieving the desired production rate. During the polymerization the co-catalyst was introduced continuously into the line for recycling the gaseous reaction mixture, at a point situated downstream of the heat transfer means. The feed rate of co-catalyst is expressed as a molar ratio of trialkylalumunium to titanium (Al/Ti), and is defined as the ratio of the co-catalyst feed rate (in moles of trialkylaluminum per hour) to the catalyst or prepolymer feed rate (in moles of titanium per hour). Tetrahydrofuran (THF) was introduced continuously into the line for recycling the gaseous reaction mixture as a solution in either n-hexane or 1-hexene at a concentration of about 1 weight percent. The feed rate of THF is expressed as a molar ratio of THF to titanium (THF/Ti), and is defined as the ratio of the THF feed rate (in moles of THF per hour) to the catalyst feed rate (in moles of titanium per hour).
In the preparation of the exemplary ethylene-1-hexene interpolymer herein, the following specific operating conditions were utilized in the process set forth hereinabove. The reactor pressure was 296 psig; the reactor temperature was 84xc2x0 C.; the fluidization velocity was 1.9 feet/second; the fluidization bulk density was 14.6; the reactor bed height was 11.5 feet; the ethylene content was 40 mole %; the molar ratio of H2/C2 was 0.421; the molar ratio of 1-hexene/C2 was 0.105; the molar ratio of TEAL (triethyluminum)/Ti (titanium) was 30.5; the molar ratio of THF/Ti was 5.8; the rate of addition of catalyst was 0.01 pounds/hour; the production rate was 186 pounds/hour; the residence time was 4.2 hours; and the residual titanium was 0.4 ppm. The resulting ethylene-1-hexene interpolymer was characterized as having a density of 0.916 g/cc, a melt index of 2.6 g/10 min, a weight % HT of about 39.2%, and a Mn of about 45,000 g/mol.
The filler useful in preparing the novel compositions of this invention include any filler material that will result in a composition from which a film that is produced is characterized by having an increased MVTR. The amount of filler utilized is any amount that is effective or sufficient to provide a composition from which there can be produced films having the increased MVTR. Preferably, the filler will be present in the composition in an amount of from about 20 to about 80% by weight, based on the total composition.
Exemplary fillers that are suitable for use herein are inorganic fillers such as calcium carbonate, talc, clay, kaolin, silica, diatomaceous earth, magnesium carbonate, barium carbonate, magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide, zinc oxide, magnesium oxide, titanium oxide, aluminum oxide, mica, glass powder, zeolite, silica clay, and the like. Preferred for use herein is a calcium carbonate, that may optionally be coated with a fatty acid. A typical calcium carbonate is that supplied by English China Clay under the registered trademark SUPERCOAT calcium carbonate, reported as being 97.6% calcium carbonate (prior to surface treatment) with a mean particle size of 1 micron (top cut of 10 microns) and surface area of 7.2 m2/g (determined by BET).
For many purposes, it may be desirable to incorporate other conventional additives with the polyethylene compositions of the present invention. For example, there may be added antioxidants, heat and light stabilizers, dyes, antistatic agents, lubricants, preservatives, processing aids, slip agents, antiblocking agents, pigments, flame retardants, blowing agents, and the like. More than one additive may be used. The additive may be present in any desired amount. Accordingly, the amount of additive utilized will depend upon the particular polyethylene and filler used and the application or usage intended for the composition and film. Compositions containing such other additives are within the scope of this invention. It is within the skill of the ordinary artisan in possession of the present disclosure to select the appropriate additive(s) and amount thereof depending on the processing conditions and end use of the composition.
The novel polyethylene compositions comprising the specified polyethylene component and the filler can be readily prepared utilizing any conventional method, and the novel films can be formed from the resultant polyethylene compositions utilizing any means known in the art. For example, polyethylene compositions can be prepared in an apparatus such as a torque rheometer, a single screw extruder or a twin screw extruder. Formation of films from the resulting compositions can be achieved by melt extrusion, as described, for example, in U.S. Pat. No. 4,880,592, or by compression molding as described, for example, in U.S. Pat. No. 4,427,614, or by any other suitable method.
In preparing the compositions of the examples herein, the polyethylene component and the filler were compounded in a Kobelco continuous mixer model NEX-T60. The mixer was operated at 400xc2x0 F., a mixing speed of 800 rpm, and a production rate of 200 lbs/hour. The polyethylene component and the filler were fed to the mixer to produce a polyethylene filler composition containing 50 wt % filler. Additionally, 100 ppm Dynamar FX 9613 processing aid (supplied by Dyneon, Oakdale, Minn.) and 150 ppm Irganox B215 antioxidant (supplied by Ciba Specialty Chemicals Corporation, Terrytown, N.Y.) were added to the polyethylene filler composition during mixing. The filler utilized was SUPERCOAT calcium carbonate, described herein.
In addition to the novel compositions, the present invention is also directed to films formed from the novel compositions that are characterized by having increased moisture vapor transmission rates. Furthermore, the physical properties of the films are not detrimentally affected as a result of incorporating the filler.
The polyethylene compositions of the present invention may be fabricated into films by any technique known in the art. For example, films may be produced by the well known cast film, blown film and extrusion coating techniques, the latter including extrusion onto a substrate. Such a substrate may also include a tie-layer. Preferred substrates include woven and nonwoven fabrics. Films produced by melt casting or blowing can be thermally bonded or sealed to a substrate using an adhesive. The ordinary artisan, in possession of the present disclosure, can prepare such films and articles containing such films without undue experimentation.
As shown hereinafter in the examples, blown film is produced from the polyethylene compositions of the present invention by introducing the composition into the feed hopper of a 2.5 inch Egan extruder with a 24/1 Length/Diameter. The film was produced using a circular 6 inch Sano die having a gap of 0.088 inch (88 mils) and dual air lips. The extrusion conditions used to process the filled polyethylene compositions (referred to as resins) were as follows:
In processing the filled polyethylene compositions, the parameters held constant were output rate (89 lb/hr=4.7 lb/hr-in die circumference) and blow-up ratio of 2.4:1. The films were to be stretched via the interdigitation method and accordingly were fabricated at a thickness of 1.3xc2x10.1 mil.
Interdigitation is a stretching process that is well known in the art whereby the filled film, while under tension, is passed between intermeshing, grooved cylinders or intermeshing disks. Machine direction stretching is accomplished by passing the film through a gear-like intermeshing cylinder pair and transverse stretching is accomplished by passing the film through a disk-like roller pair. Each point of contact with the grooves or disks applies localized stress to the film. It is at these points that the film stretches. The resulting stretched film consists of narrow, parallel bands where stretching has occurred separated by bands of unstretched film. The amount of stretching is governed by the amount of interengagement between the grooved cylinder pair or the intermeshing disk pair. Pores in films stretched by this process are found in the stretched bands. When films are stretched biaxially by the interdigitation process, a crosshatched pattern of stretched bands is produced. The examples shown here of films stretched biaxially at room temperature by the interdigitation method were prepared at Biax FiberFilm Corporation, Greenville, Wis. Further information on the interdigitation method may be found in U.S. Pat. No. 4,116,892 and PCT WO 00/23255. The stretch ratio was held constant at a 1.1xc3x97 machine direction stretch followed by a 1.125xc3x97 transverse direction stretch. The stretch ratio is determined by drawing a 1 inch diameter circle on the film and then passing this film through the intermeshing grooved cylinder pair or intermeshing disk pair. The circle diameter is then again measured, in the direction, yielding the stretch ratio.
Additionally, the present invention is directed to articles of manufacture incorporating the novel compositions and novel films, of the present invention. Such articles include, but are not limited to, garments, diapers, sanitary napkins, medical protective garments, surgical incise drapes, transdermal patches, wound care bandages and dressings, intravenous site dressings, and ostomy site dressings, and others. The articles can be produced utilizing any suitable technique.